Coated silicon comprising material for protection against environmental corrosion

ABSTRACT

In accordance with an embodiment of the invention, an article is disclosed. The article comprises a gas turbine engine component substrate comprising a silicon material; and an environmental barrier coating overlying the substrate, wherein the environmental barrier coating comprises cerium oxide, and the cerium oxide reduces formation of silicate glass on the substrate upon exposure to corrodant sulfates.

GOVERNMENT RIGHTS

The invention was made in part under contract number NAS3-01135 awardedby the Government (NASA). Accordingly, the Government has certain rightsin this invention.

FIELD OF THE INVENTION

This invention relates to protecting materials comprising silicon fromenvironmental corrosion, such as that experienced in the hostile thermalenvironment of a gas turbine engine. More particularly, the inventionrelates to an environmental barrier coating (EBC) system for use onsilicon comprising substrates for providing protection againstenvironmental corrosion.

BACKGROUND OF THE INVENTION

Higher operating temperatures for gas turbine engines are continuouslysought in order to increase efficiency. However, as operatingtemperatures increase, the high temperature durability of the componentswithin the engine must correspondingly increase. In this regard,materials comprising silicon, particularly those with silicon carbide(SiC) as a matrix material or a reinforcing material, are considereduseful for high temperature applications, such as for combustor andother hot section components of gas turbine engines.

However, some silicon substrates may recede and lose mass as a result offormation of volatile Si species, particularly Si(OH)_(x) and SiO whenexposed to high temperature, aqueous environments, thus necessitatingthe use of a protective coating thereon. Accordingly, methods such asdescribed in U.S. Pat. Nos. 5,985,470, 6,444,335, 6,410,148 and6,759,151, the contents of each of which are incorporated by reference,have addressed shortcomings concerning the use of such siliconsubstrates by providing an environmental barrier coating (EBC) over thesubstrate. The EBCs inhibit formation of volatile silicon species,Si(OH)_(x) and SiO, thereby reducing recession and mass loss. A thermalbarrier coating (TBC) typically comprising yttria stabilized zirconiamay also be employed as an outer layer to the EBC depending upon theoperating conditions employed.

While the current state of the art EBCs, which are typically multi-layerEBCs, may be effective in preventing water vapor recession of theceramic matrix composite (CMC) substrate, both BSAS, SiC and some rareearth silicates such as some silicates disclosed in U.S. Pat. No.6,759,151 may be susceptible to silicate glass formation when in contactwith sulfate salt deposits as operating conditions continue to increase.

Accordingly, there is a need to reduce the silicate glass formation ratein materials comprising silicon, particularly in EBC materials.Embodiments of the present invention satisfy this need and others.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with an embodiment of the invention, an article isdisclosed. The article comprises a gas turbine engine componentsubstrate comprising a silicon material; and an environmental barriercoating overlying the substrate, wherein the environmental barriercoating comprises cerium oxide, and the cerium oxide reduces formationof silicate glass on the substrate upon exposure to corrodant sulfates.

In accordance with another embodiment of the invention, an articlecomprises a gas turbine engine component substrate comprising a siliconmaterial. The article further comprises a bond layer comprising asilicon material and overlying the substrate; a first layer comprisingmullite and overlying the bond layer; a second layer comprising anbarium strontium aluminosilicate and overlying the first layer; and athird layer consisting of cerium oxide and overlying the second layer.The cerium oxide reduces formation of silicate glass on the substrateupon exposure to corrodant sulfates.

In accordance with another embodiment of the invention, a gas turbineengine component is disclosed. The component comprises a substratecomprising a silicon material; and at least one layer on the substrateselected from the group consisting of: i) an environmental barriercoating comprising cerium oxide, and ii) a dual layer of bariumstrontium aluminosilicate and a layer consisting of cerium oxideoverlying the barium strontium aluminosilicate; wherein the cerium oxidereduces formation of silicate glass on the substrate.

In accordance with a further embodiment of the invention, a gas turbineengine component comprises a substrate comprising a silicon material.Cerium oxide is deposited directly on the substrate or admixed with thesilicon material of the substrate, wherein the cerium oxide reducesformation of silicate glass on the substrate upon exposure to corrodantsulfates.

In accordance with another embodiment, a method of reducing silicateglass formation on a gas turbine engine component is disclosed. Themethod comprises providing a substrate of the gas turbine enginecomponent, wherein the substrate comprises a silicon material; anddepositing cerium oxide directly on the substrate or admixing ceriumoxide with the silicon material of the substrate. The cerium oxidereduces formation of silicon glass on the substrate upon exposure tocorrodant sulfates.

In accordance with another embodiment, a method of reducing silicateglass formation on a gas turbine engine component comprises providing asubstrate of the gas turbine engine component, wherein the substratecomprises a silicon material. The method further comprises depositing abond layer comprising a silicon material and overlying the substrate;depositing a first layer comprising mullite and overlying the bondlayer; depositing a second layer comprising an environmental barriercoating overlying the first layer; and depositing a layer consisting ofcerium oxide overlying second layer or admixing cerium oxide with bariumstrontium aluminosilicate. The cerium oxide reduces formation of siliconglass on the substrate upon exposure to corrodant sulfates.

Other features and advantages will be apparent from the following moredetailed description, taken in conjunction with the accompanyingdrawing, which illustrate by way of example the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a gas turbine engine componentformed of a material comprising Si and having an environmental barriercoating thereon, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Applicant has determined through testing that silicon comprisingmaterials may adversely react with deposited sulfates. For example, thismaterial may react with Na₂SO₄ at temperatures such as about 1700° F.(927° C.) and higher, and MgSO₄/CaSO₄ may react at temperatures such asabout 2200° F. (1204° C.) and higher. This reaction causes corrosion andforms a silicate glass from the silicon comprising material. Deeppitting and CO evolution can degrade the performance of the siliconcomprising material. Additionally, this may prevent EBCs from acting asoptimal water recession barriers for silicon comprising substrates assilicate glass has a high recession rate.

Embodiments of the present invention improve upon prior coated siliconcomprising materials, particularly EBC systems used upon siliconcomprising material substrates. These embodiments are generallyapplicable to components that operate within environments of relativelyhigh temperatures, and are thus subjected to thermal cycling, stresses,oxidation, and corrosion. Examples of such components include, but arenot limited to, combustor components, blades, shrouds, flaps, seals,vanes, and other components of gas turbine engines.

Referring to FIG. 1, an EBC system 10 of a first embodiment is shown.The EBC system 10 includes an EBC 12, and a surface region 16 orsubstrate of a component 18. The component 18, or at least the surfaceregion (substrate) 16 of the component 18, is formed of a siliconcomprising material (particularly those for articles exposed to hightemperatures), such as SiC/SiC ceramic matrix composites (CMC). However,embodiments of the invention are generally applicable to other materialscomprising silicon. Examples of silicon comprising materials include,but are not limited to, those with a dispersion of silicon carbide,silicon carbide and/or silicon particles as a reinforcement material ina metallic or nonmetallic matrix. Also included are those having asilicon carbide, silicon aluminum oxynitride, silicon nitride and/orsilicon comprising matrix, and particularly composite materials thatemploy silicon carbide, silicon nitride and/or silicon as both thereinforcement and matrix materials (e.g., SiC/SiC ceramic matrixcomposites (CMC)). Silicon comprising materials further include metalsilicides including, but not limited to, molybdenum and niobiumsilicides. Thus, according to embodiments of the invention, the siliconcomprising substrate 16 may be a silicon comprising ceramic material as,for example, silicon carbide, silicon nitride, silicon carbon nitride,silicon oxynitride and silicon aluminum oxynitride. In accordance withone embodiment, the silicon comprising substrate 16 comprises a siliconcomprising matrix with reinforcing fibers, particles and the like and,more particularly, a fiber reinforced silicon based matrix. Particularlysuitable ceramic substrates are a silicon carbide coated silicon carbidefiber-reinforced silicon carbide particle and silicon matrix, a carbonfiber-reinforced silicon carbide matrix and a silicon carbidefiber-reinforced silicon nitride matrix. Particularly usefulsilicon-metal alloys for use as substrates 16 for the article ofembodiments of the invention include molybdenum-silicon alloys,niobium-silicon alloys, and other Si comprising alloys having acoefficient of thermal expansion compatible with the other layer(s)described herein.

In accordance with one embodiment, the surface region or substrate 16 ofthe component 18 is protected by the multilayer EBC system 10 thatincludes the EBC 12 for providing environmental protection to thecomponent 18. Optionally, a top coat or conventional thermal barriercoating (not shown), as well as optional conventional intermediatelayer(s) (not shown) may be provided on top of the EBC 12 for providingfurther thermal insulation to the underlying layers depending upondesired operational temperatures.

The multi-layered EBC 12 of the embodiment shown in FIG. 1, preferablyhas four layers, as shown therein. These four layers may include a bondlayer 22, a first layer 24, a second layer 26 and a third layer 28. Thebond layer 22 overlays the substrate 16 of the component 18 andpreferably comprises silicon, such as at least one of silicon metal andsilicon dioxide. This bond layer 22 is useful to improve oxidationresistance of the surface region 16 and enhance bonding between thefirst layer 24 and the surface region 16, particularly if the surfaceregion 16 comprises silicon carbide or silicon nitride. A suitablethickness for the bond layer 22 is about 12.5 to about 250 micrometers.Suitable materials for bond layer 22 also include those described in theafore-referenced U.S. Pat. No. 6,410,148. For example, bond layer 22 caninclude a silicon metal or a SiO₂ layer.

The first layer 24 is located on the bond layer 22 and comprisesmullite. This mullite comprising first layer 24 serves to adhere thesecond layer 26 to the surface region 16, while also preventinginteractions between the second layer 26 and the silicon comprisingsurface region 16 at elevated temperatures. The first layer 24 may alsocomprise BSAS for less demanding applications, e.g. temperatures belowabout 1300° C. The addition of BSAS to the layer 24 is also relativelycompatible with the silicon comprising surface region 16 in terms ofhaving a CTE of about 5.27 ppm/° C., as compared to a CTE of about 4.9ppm/° C. for SiC/SiC CMC. Preferably, first layer 24 comprisesmullite-barrium strontium aluminosilicate (BSAS) in an amount of betweenabout 40 to 80 wt. % mullite and between about 20 to 60 wt. % BSAS. Asuitable thickness range for the mullite comprising first layer 24 isabout 25 to about 250 micrometers.

The second layer 26 overlies the first mullite comprising layer 24 andtypically comprises BSAS and may consist essentially of BSAS. This layerprovides excellent environmental protection and thermal barrierproperties due to its low thermal conductivity. Particularly, BSAS canserve as an environmental barrier to the underlying mullite comprisinglayer 24, which would exhibit significant silica activity andvolatilization if exposed to water vapor at high temperatures.Additionally, BSAS is physically compliant with a SiC comprisingsubstrate, such as that suitable for surface region 16, and isrelatively compatible with the mullite comprising layer 24 and thesilicon comprising surface region 16 in terms of CTE. A suitablethickness range for layer 26 is about 25 to about 500 micrometers,depending upon the particular application.

The second layer 26 may alternatively or additionally comprise a rareearth silicate such as described in U.S. Pat. No. 6,759,151. Forexample, rare earth silicates, such as those described in U.S. Pat. No.6,759,151 may be employed as in place of the BSAS described herein oradmixed therewith. As a further example, rare earth silicates include,but are not limited to, RE₂O₃, SiO₂, 2RE₂O₃.3SiO₂, RE₂O₃.2SiO₂, andcombinations thereof, where RE is a rare earth element selected from thegroup consisting of Sc, Dy, Ho, Er, Tm, Yb, Lu, Eu, Gd, Tb andcombinations thereof.

In the embodiment shown in FIG. 1, the EBC 12 also comprises a ceriumoxide (CeO₂) layer 28 (third layer 28) located on top of the secondlayer 26. A suitable thickness range for this layer is between about 5to about 75 micrometers, also depending upon the particular application.Alternatively, the cerium oxide may be admixed with the second layer 26.For example, cerium oxide may be admixed in any suitable amounts such asin a 50:50-weight percentage with the constituents of the second layer26.

In alternate embodiments of the invention, the EBC 12 may comprise thesecond layer 26 deposited directly on the substrate comprising siliconand the third layer 28 deposited on the second layer 26. Alternatively,an admixed layer of the second layer and the third layer 28 may bedeposited on the substrate comprising silicon. It should be noted thatin these embodiments, bond layer 22 may also be employed and depositeddirectly on the substrate comprising silicon with the additional layersdeposited on the bond layer 22.

-   -   Thus, in accordance with embodiments of the invention, a single        layer, EBC of BSAS and CeO₂ admixed therewith, as described        above, may be used to provide environmental protection to the        underlying silicon comprising material. This embodiment is        particularly useful for operating temperatures below about        3000° F. (1371° C.), Alternatively, a CeO₂ layer, as described        above, may be deposited on top of the BSAS layer.

The layers 22, 24, 26 and 28 can be individually deposited by air andvacuum plasma spraying (APS and VPS, respectively), though it isforeseeable that deposition could be performed by other techniques, suchas chemical vapor deposition (CVD) and high velocity oxy fuel (HVOF). Aheat treatment may also be performed after deposition of the individuallayers to relieve stresses created during cooling from elevateddeposition temperatures.

Accordingly, Applicant has advantageously determined that deposition ofthis high melting point acidic compound, CeO₂, will result indissolution into the basic corrosion melt and increase the eutectictemperature and/or decrease the basicity of the solution. This willadvantageously slow and/or stop the silicate glass formation on thesurface of the silicon comprising material by raising the melting pointor decreasing the basicity of the silicate glass of the corrodant mix.

While various embodiments are described herein, it will be appreciatedfrom the specification that various combinations of elements, variationsand improvements therein may be made by those skilled in the art, andare within the scope of the invention.

1-14. (canceled)
 15. A method of reducing silicate glass formation on agas turbine engine component comprising: providing a substrate of thegas turbine engine component, wherein the substrate comprises a siliconmaterial; and depositing cerium oxide directly on the substrate oradmixing cerium oxide with the silicon material of the substrate;wherein the cerium oxide reduces formation of silicon glass on thesubstrate upon exposure to corrodant sulfates.
 16. A method of reducingsilicate glass formation on a gas turbine engine component comprising:providing a substrate of the gas turbine engine component, wherein thesubstrate comprises a silicon material; depositing a bond layercomprising a silicon material and overlying the substrate; depositing afirst layer comprising mullite and overlying the bond layer; depositinga second layer comprising barium strontium aluminosilicate overlying thefirst layer; and depositing a layer consisting of cerium oxide overlyingsecond layer or admixing cerium oxide with the barium strontiumaluminosilicate; wherein the cerium oxide reduces formation of siliconglass on the substrate upon exposure to corrodant sulfates.
 17. Themethod of claim 16, wherein each layer is deposited by a method selectedfrom the group consisting of: air plasma spray, vacuum plasma spray,chemical vapor deposition and high velocity oxy fuel.